.243 mate.
It's the closest thing to an all rounder you can get and still find cheap factory ammo anywhere. You'll fall short (legally) for Sambar in Vic, but solving that problem with a 7mm-08 will make ammo marginally harder to find and certainly more expensive plus arguably leaving you overgunned for small stuff at least with factory ammo.
I bought a .223 as my first centrefire with exactly the same "shoot all day and not go broke" mindset, and wished I'd gone the .243 ever since. It's not that much more expensive but exponentially more versatile.